Metrics collectors generally don't need the daemon to prime the stats
with something to compare since they already have something to compare
with.
Before this change, the API does 2 collection cycles (which takes
roughly 2s) in order to provide comparison for CPU usage over 1s. This
was primarily added so that `docker stats --no-stream` had something to
compare against.

Really the CLI should have just made a 2nd call and done the comparison
itself rather than forcing it on all API consumers.
That ship has long sailed, though.

With this change, clients can set an option to just pull a single stat,
which is *at least* a full second faster:

Old:
```
time curl --unix-socket
/go/src/github.com/docker/docker/bundles/test-integration-shell/docker.sock
http://./containers/test/stats?stream=false\&one-shot=false > /dev/null
2>&1

real0m1.864s
user0m0.005s
sys0m0.007s

time curl --unix-socket
/go/src/github.com/docker/docker/bundles/test-integration-shell/docker.sock
http://./containers/test/stats?stream=false\&one-shot=false > /dev/null
2>&1

real0m1.173s
user0m0.010s
sys0m0.006s
```

New:
```
time curl --unix-socket
/go/src/github.com/docker/docker/bundles/test-integration-shell/docker.sock
http://./containers/test/stats?stream=false\&one-shot=true > /dev/null
2>&1
real0m0.680s
user0m0.008s
sys0m0.004s

time curl --unix-socket
/go/src/github.com/docker/docker/bundles/test-integration-shell/docker.sock
http://./containers/test/stats?stream=false\&one-shot=true > /dev/null
2>&1

real0m0.156s
user0m0.007s
sys0m0.007s
```

This fixes issues with downstreams ability to use the stats API to
collect metrics.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>